1. Field of the Invention
The present invention relates to a resonant element including stripline resonators provided on a dielectric substrate, and to a method for manufacturing the resonant element.
2. Description of the Related Art
A resonant element including stripline resonators has been provided on a dielectric substrate to function as a filter or a balun (e.g., see Japanese Unexamined Patent Application Publication No. 2000-22404 and Japanese Unexamined Patent Application Publication No. 2004-147300).
The resonant elements disposed in Japanese Unexamined Patent Application Publication No. 2000-22404 and Japanese Unexamined Patent Application Publication No. 2004-147300 include a plurality of laminated dielectric substrate layers and principal-surface electrodes provided between the dielectric substrate layers. The resonant elements include a coupling adjusting electrode that faces a plurality of principal-surface electrodes via the dielectric substrate layers, so that the degree of coupling between the resonators is increased. In the configuration according to Japanese Unexamined Patent Application Publication No. 2000-22404, the respective dielectric substrate layers have the same permittivity, and almost the entire coupling degree is set by the capacitance between the coupling adjusting electrode and the principal-surface electrodes. On the other hand, in the configuration according to Japanese Unexamined Patent Application Publication No. 2004-147300, the plurality of laminated dielectric substrate layers have different permittivities, and the coupling degree is adjusted by adjusting the permittivities. Such resonant elements are manufactured by laminating a plurality of dielectric green sheets and an electrode paste a plurality of times and by sintering the laminate at one time. In each manufactured lot, a plurality of resonant elements are formed on a large laminate sheet and the respective resonant elements are obtained through dicing after sintering the laminate sheet.
The above-described sintering causes variations in shrinkage and composition of the respective dielectric green sheets and variations in quality in respective manufactured lots, so that not all of the resonant elements in the same manufactured lot satisfy a desired frequency characteristic. Particularly, in a resonant element including multistage resonators coupled to each other, variations in coupling degree among the resonators cause the frequency characteristic of the product to deviate from a necessary frequency characteristic. Accordingly, it has been required that variations in the frequency characteristic among manufactured lots be suppressed.